A scroll compressor mainly has a structure shown in FIG. 12 and FIG. 13. Stationary scroll (hereinafter referred to as scroll) 52 and slewing scroll (hereinafter referred to as scroll) 53 are respectively shaped such that scroll-like wraps 50 and 51 are raised from end plates 52A and 53A so as to be substantially perpendicular to the end plates. In scroll 52 and scroll 53, wraps 50 and 51 are engaged to each other to have compression space 54 therebetween. A capacity of compression space 54 is decreased by a circular orbit motion of scroll 53 while compression space 54 moves from an outer circumference leading to inhale opening 55 to a center part leading to discharge opening 56. Thus, fluid is inhaled, compressed, and discharged.
Scrolls 52 and 53 are made by metal mainly of iron or aluminum. After they are formed by a casting or the like, the side faces of wraps 50 and 51 which slide relative to each other are finished by being cut by end mill 57 having 2 to 10 cutting blades. In this manner, a required performance is secured.
Methods for finishing an involute curve-shaped scroll wrap by an end mill are disclosed by Japanese Patent Unexamined Publication No. H04-284509, Japanese Patent Unexamined Publication No. H07-164231, and Japanese Patent Unexamined Publication No. 2000-205155, for example. According to these publications, the feed rate of the end mill is controlled depending on the curvature radius of the involute curve. Any of these methods secures the plane roughness of a machined face.
However, in the case of cutting and finishing by machining by end mill 57, the side walls of wraps 50 and 51 have an accuracy that depends not only on the machining accuracy of end mill 57 but also on a runout due to an error at which end mill 57 is attached, and also on the machining conditions. Thus, it is difficult to secure and manage a favorable accuracy in a stable manner, and a poor surface roughness is the result. In order to prevent leakage of compressed gas by reducing the clearance between the side faces of scrolls 52 and 53 while they are engaged to each other, it is necessary that the dimensional accuracies of the side faces be improved.
Furthermore, each of end plates 52A and 53A has a large surface roughness because they are cut by end mill 57 together with the side faces of wraps 50 and 51. The rough surface includes a sharp tip end at each convex. The sharp tips cause a sliding loss and a leakage loss of compressed gas, and thus the efficiency of the compressor is insufficient and deteriorates easily.
Furthermore, the machining by end mill 57 is generally performed with 20,000 revolutions per minute in order to suppress the abrasion of the blade edge. Thus, in order to secure the machining efficiency, the feed amount per one rotation needs to be increased. If the feed amount per one rotation is increased, uneven machining is caused in a regularly-repeated manner due to the existence of a part of end mill 57 having the cutting blade and a part having no cutting blade. This uneven machining is caused with a pitch inversely proportional to the number of the cutting blades. Furthermore, an error caused when end mill 57 is attached also causes a regularly-repeated runout, causing swells in the longitudinal direction of the side faces of wraps 50 and 51. These swells cause minor vibrations in scroll 53 while the compressor is being operated, thus increasing noise.
Furthermore, when corners of side faces of wraps 50 and 51 and corners of end plates 52A and 53A at the boundary are abraded by sliding contact with the corner of the tip end of the outer circumference of end mill 57, the shapes of corners of side faces of wraps 50 and 51 and the shapes of corners of end plates 52A and 53A are changed. In consideration of this, the inner and outer corners of the upper end face of an upper wrap engaged with a lower wrap need to be provided with a large chamfer and thus a space provided therebetween is increased. This increases the leakage of compressed gas, deteriorating the efficiency of the scroll compressor.